The inside wall of a molten metal container, such as molten pig iron vessel, torpedo car, a molten steel ladle, ladle refinery, tundish, etc. has been lined with refractory so as to give it slag resistance.
With the introduction of new processes, such as dephosphorizing, desilicifying or continuous casting, degassing refining, etc., as a result of the recent development of the pig iron and steel producing technology, the lining life of the refractory of the molten metal container, particularly of the part most susceptible to the direct attack by slag, for example, the slag line, charging wall, charging part of the bottom (the part at which molten steel and slag are brought into direct contact), slagging wall, etc., has been reduced.
In the location most susceptible to slag attack of the lining refractory of a molten steel ladle, the temperature is particularly high and the chemical reaction with oxides, such as CaO, SiO.sub.2, FeO, MnO, etc. in the slag is conspicuous.
The lining refractory is worn as a result of structural or thermal spalling arising from variation of the crystalline phase of the matrix of the bricks forming the lining due to infiltration of the slag components, the generation of new mineral phases, by the infiltrated components, and unusual expansion and contraction due to repetition of heating and cooling.
The introduction of the afore-described new processes has brought about the elevation of temperature of the molten steel and prolongation of its residence time in the ladle. Thus the location of the lining material most susceptible to slag attack is subjected to a very high temperature not only on its surface but also to a considerable depth.
The slag components, with the viscosity thereof being reduced by high temperature, have greater reactivity to the refractory, such components and reaction products being permitted to infiltrate deep into the interior of the refractory, resulting in structural spalling thereby greatly reducing the lining life of the refractory.
As the lining refractory for use in the location most susceptible to slag attack in the molten steel ladle, a basic refractory, such as dolomite, magnesia, etc., has been used heretofore for its resistance to corrosion by basic slag. However, no satisfactory lining life has been attained due to exfoliation arising from structural or thermal spalling and adhesion of slag or steel to the surface of the refractory.
With a view to obviating said defects of the basic refractory lining applied to the location most susceptible to slag attack, refractory consisting of magnesia, forsterite and silicon carbide has been used (Japanese Laid-Open Publication No.SHO-49-5407). However, no improvement in the corrosion resistance has been obtained since SiC is thermally decomposed into Si and C during use at high temperatures, the generated carbon being very unstable and liable to be oxidized. Thus the characteristics of the basic refractory have not been utilized due to increased melting loss.
Refractory bricks comprising magnesia, silicon carbide and carbon with a metallic material (a steel plate) buried therein were proposed as the ladle lining refractory (Japanese Laid-Open Publication No.SHO-51-95936). Under the oxidizing atmosphere of the molten steel ladle, however, carbon was oxidized and was therefore useless in precluding slag infiltration, no sufficient improvement of the lining life being obtainable due to structural spalling. In addition, since the steel receiving temperature of the molten steel ladle was higher than 1500.degree. C., the metal plate was softened and deformed, the peripheral refractory was weakened, thermal spalling was liable to be generated due to the difference in the thermal characteristics, for example, thermal expansion coefficient, between the metal plate and the refractory, causing exfoliation and resulting in infiltration of slag or steel.
Furthermore, the buried metal plate reduces the compactness of the refractory since it prevents movement of the particles at press-molding, resulting in poor corrosion resistance. Thus the buried metal plate, which is used only as a supplemental means without improving the characteristics of the refractory itself, brings no fundamental solution to the problem of the improvement of the lining life thereof.